<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
   <input type="text" id="u1">
   <select id="opt">
    <option value="+">+</option>
    <option value="-">-</option>
    <option value="*">*</option>
    <option value="/">/</option>
   </select>
   <input type="text" id="u2">
   <span onclick="count()">=</span>
   <span id="res"></span>
</body>
<script>
    function count(){
        let u1 =document.getElementById("u1").value;
        let opt =document.getElementById("opt").value;
        let u2 =document.getElementById("u2").value;
        document.getElementById("res").innerHTML=eval(u1+opt+u2)
    }
</script>
</html>